This invention relates to processes for the hydrolysis of alkoxy-functional silanes, and more particularly, to methods for the catalytic hydrolysis of alkoxysilanes.
Low molecular weight, silanol-stopped (hydroxyend blocked) linear diorganopolysiloxanes are well-known materials in the organosilicon art. One well-known commercially available material is a low molecular weight, silanol-stopped dimethylpolysiloxane fluid which has a variety of utilities including use as a rubber processing aid, used in silicone hard coats, paper release coatings, release emulsions and room temperature vulcanizable silicone rubbers.
The silanol-stopped diorganopolysiloxane fluids have been prepared, for example, by the carefully controlled hydrolysis of diorganodihalosilanes, by neutralization of metal ester and blocked linear diorganosiloxanes, by reaction of diorganopolysiloxanes with water and an organic nitrile in the presence of a basic catalyst, by reaction of diorganopolysiloxanes with aqueous solutions of monobasic acids, by the hydrolysis of trimers, such as hexamethylcyclotrisiloxane catalyzed with an acid clay catalyst as described in U.S. Pat. No. 3,309,390, and the like. The prior art also teaches the water hydrolysis of dialkyldialkoxysilanes with excess water heated at the boiling point using rigorously neutral conditions, and acid catalysts have been used for the hydrolysis of dialkyldialkoxysilanes for making higher molecular weight polymeric silicones as disclosed in U.S. Pat. No. 2,384,384. Alkoxysilanes have also been hydrolyzed using strongly acidic cation exchange resins as the hydrolysis catalyst as disclosed in U.S. Pat. No. 3,304,318.
Generally, it has been difficult to hydrolyze diorgano di-functional silanes, such as dialkyldichlorosilanes or dialkyldialkoxysilanes, to produce low molecular weight, linear silanol-stopped diorganopolysiloxane fluids free of cyclic compounds. Most of the prior art methods discussed above result in mixtures of the desired materials with high molecular weight linear polysiloxanediols or polysiloxane cyclics or both. It is very difficult or impossible to separate the desired materials from these high molecular weight and cyclic components. Furthermore, many of the prior art processes, wherein the silanol-stopped polysiloxanes are made from dialkoxysilanes, produce oligomers having a high alkoxy content, that is, may of the end groups of the polymers are endcapped with alkoxy groups. It has also been difficult to prepare low molecular weight silanol-stopped polysiloxanes having controlled chain lengths by prior art catalyst hydrolysis processes.
Trifunctional alkoxysilanes, RSi(OR').sub.3, wherein R is methyl or phenyl, have been hydrolyzed by a process described in U.S. Pat. No. 3,642,693. The hydrolysis process of the trialkoxysilanes is carried out under weakly basic conditions using saturated aqueous solutions of barium hydroxide, calcium hydroxide and strontium hydroxide prepared from the corresponding Group IIa metal oxides and water. There appears to be no reference or suggestion in U.S. Pat. No. 3,642,693 of the hydrolysis of the dialkoxysilanes or of the formation of low molecular weight silanol-stopped diorganopolysiloxane fluids substantially free of cyclic compounds.
In J. Am. Chem. Soc. 76, 3408-3414 (1954), the rates of silanol condensation reactions in methanol with both acidic and basic catalysis are measured. However, even though the condensation reaction of ##STR1## groups with ##STR2## groups is described, it appears that the method has not been used to reduce the alkoxy content of a siloxane made from alkoxysilane. Generally, polysiloxane silanols have been made from alkoxysilanes using a moderate excess of water in a one-step hydrolysis, and it appears that except for U.S. Pat. No. 4,032,557, there has been no attempt to reduce the alkoxy levels below the resulting equilibrium value of about 1 to about 3%. U.S. Pat. No. 4,032,557, describes steam distillation in the presense of chlorosilanes to reduce the residual methoxy chain stopping from 2% to 0.3% of the chain ends.
Accordingly, it is an object of the present invention to provide a process for the catalytic hydrolysis of alkoxysilanes.
It is another object of the present invention to provide a process for the catalytic hydrolysis of alkoxysilanes, including diorganodialkoxysilanes, which results in a high yield of low molecular weight, linear polysiloxane diols which have little or no cyclic by-products.
Another object of the present invention is to provide a process for the catalytic hydrolysis of diorganodialkoxysilanes wherein the diorganodialkoxysilane is extensively converted to silanol end-stopped oligomers averaging about 2 to about 8 siloxane units.
It is also an object of the present invention to provide a process for preparing silanol-stopped, linear polysiloxanes from alkoxysilanes, e.g., dialkoxysilanes, wherein the alkoxy content of the product is substantially reduced.
Another object of the present invention is to provide a method for controlling the chain length of low molecular weight, linear, silanol-stopped polysiloxanes made by the catalytic hydrolysis of alkoxysilanes, such as, dialkoxysilanes.
Another object of the present invention is to provide a simplified process for preparing and isolating silanol-stopped, low molecular weight, linear polysiloxanes, thereby eliminating the complex separation techniques necessitated by many of the prior art methods.
Still another object of the present invention is to provide stabilized crystalline tetramethyldisiloxanediol and dimethyldisiloxanediol prepared by the catalytic hydrolysis of dimethyldimethoxysilane.
Other objects and advantages of the present invention will become apparent from the following detailed description.